Solid state imaging devices include an array of pixel cells, which converts light energy into electrical signals. Each pixel cell contains a photosensor for converting a respective portion of a received image into an electrical signal. The electrical signals produced by the photosensors are processed to render a digital image.
Photosensors are sensitive to light in the visible spectrum and convert that light into a light intensity signal value. To capture color images, the spectral components of incident light must be separated and collected. To this end, a color filter array (CFA) is often associated with the pixel array. Color filters are imposed in front of the pixel cells and over the photosensors and are arranged in a predetermined pattern. The color filters of the color filter array are typically pigmented or dyed material that will only pass a narrow color band of visible light, e.g., red, blue, or green. A common example of a color filter array pattern is the tiled color filter array illustrated in U.S. Pat. No. 3,971,065, and commonly referred to as “the Bayer pattern” color filter. The color filter array allows what would otherwise be black and white image sensors to produce color images. Other color filter array patterns are also known.
As imaging devices are used in smaller and smaller applications, there is a need to decrease the stack height of the imaging array of the imaging device, requiring the use of a recessed color filter array. A recessed color filter array is formed in a recess of a substrate layer which is provided over photosensors. The recessed arrangement prevents the color filter array from extending above a desired pixel array stack height.
FIG. 1 shows a simplified, partial cross section of a recessed color filter array 14 in an imaging device 100. The imaging device 100 includes a substrate layer 10 having a recess 12 etched or otherwise formed therein. While FIG. 1 only shows a portion of a substrate layer 10 of a single imaging device 100, i.e., a single die, it should be understood that the processing steps may be and generally are performed on multiple dies of a wafer at the same time. A nitride layer 16, for example, silicon nitride (Si3N4), is formed on the surface of the substrate layer 10 including the recess 12.
The color filter array 14 generally has several different filter colors, such as red, green and blue, or cyan, magenta, yellow, and black, or other combination of filter colors arranged in a pattern. Filters of the same color are formed by a series of steps including coating a filter material, such as a colored polyimide, exposing it through a pattern mask, and developing the filter material into a pattern of filters of the desired color. For example, to form a Bayer pattern filter, a liquid green filter material is coated onto the substrate layer 10 including the recess 12 by a method such as spin coating. The green filter material is exposed, for example, to ultraviolet light through a mask and developed into a pattern of green color filters. The green color filters are then cured. The green filter material is developed into a pattern, for example, by using a solvent to remove portions of the green filter material that are not developed. The green filter material is then cured again. The coating, exposing, developing, and curing steps are repeated for the blue filter material to form blue filters and then again for red filter material to form red filters to complete the color filter array 14. Microlenses (not shown) to focus the light onto the photosensors may be formed on top of the color filter array 14.
As illustrated in FIG. 1, one problem associated with formation of a conventional recessed color filter array 14 is that the color filter array 14 may have a non-uniform thickness and may be thicker at the edges of the color filter array 14 than at the center. The majority portion of the color filter array 14 located away from the edge of the recess 12 is formed to a relatively uniform thickness d. The distance from the edge of the recess 12 to the portion of the color filter array 14 at the relatively uniform thickness d is known as the settling distance s. The portion of the color filter array 14 spanning the settling distance s slopes down from the edge of the recess 12 towards the portion of the color filter array 14 having the relatively uniform distance d.
This lack of uniformity in color filter array 14 thickness can cause problems in the imaging device 100, such as making the pixel output signals for underlying pixels associated with the edges of array 14 non-uniform and providing a poor foundation for the microlenses. In addition, an uneven thickness of color filter array 14 can cause imaging efficiency reduction by creating additional fixed pattern noise or shading effects. Specifically, fixed pattern noise, which is a spatial variation in pixel output values under uniform illumination, results from the variation of color filter material. An undesirable shading effect occurs as the result of non-planarity of the color filter array over the entire surface of the array. Thus, having a color filter array 14 with a more uniform thickness can advantageously help to create a solid foundation for microlenses, reduce fixed pattern noise, and decrease undesirable noise shading.